1. Field of the Invention
This invention relates to digital circuits wherein the source of one metal-semiconductor field-effect transistor (MESFET) is directly electrically connected to the drain of a second MESFET, and the MESFETS are driven in complementary fashion by logic signals controlled by an inverter which includes a third MESFET. The invention further relates to the above described circuit including a means for disabling the logic input to place the circuit in a standby, low power state. The preferred embodiment uses predominately depletion mode MESFETS.
2. Prior Art
Various schemes are known for driving two FETS in complementary fashion. In U.S. Pat. No. 4,347,447 to Proebsting, two metal-oxide semiconductor FETS (MOSFETS) are driven in complementary fashion by an inverter (see transistor 16, FIG. 1 therein). In U.S. Pat. No. 4,363,978 to Heimbigner two enhancement mode MOSFETS serve to separately invert logic signals and separately drive two enhancement mode MOSFETS in complementary fashion. In U.S. Pat. No. 4,380,709, issued to Au, enhancement mode MOSFETS are driven in complementary fashion by separate inverters, each of the inverters employing a depletion mode MOSFET.
Additionally, the circuits disclosed in the patents to Proebsting, Heimbigner and Au each include means for placing the circuits in a third or standby state. In Proebsting, a disable signal is provided by using two depletion mode MOSFETS. When the circuit of Proebsting is disabled, the output pin is left floating and a high impedance will be present at the logic output pin. In Heimbigner a "float" mode exists in which the output circuit will neither supply current to the output load or switch the output load to ground, and therefore other drivers can control the logic state of the output load. Heimbigner provides float and not-float signal through separate enhancement mode MOSFETS.
In Au, a disable circuit employs four MOSFETS (including depletion and enhancement types) and leaves the output floating when activated.
For many low power, high speed digital operations, MESFETS offer advantages over MOSFET circuits. This is particularly true when the substrate is GaAs instead of silicon due to the high mobility of electrons in GaAs.
Heretofore, a simple MESFET circuit for driving two MESFETS in complementary fashion has not been disclosed. Likewise, an output buffer providing a low power standby state in addition to the simple MESFET circuit indicated above has also not been previously disclosed. The low power tristate function is an important system feature no previously disclosed for GaAs buffer circuits.